January 17, 2011 - Round And Round They Go

Round and Round They Go The fact that stars appear to be fixed in their positions in the night sky is partly the result of our relatively short life spans. All stars are in motion, most of them imperceptibly, but over time and with increasingly sophisticated observational tools it has become possible to map just where and how fast the stars are moving. Dutch astronomer Jacobus Kapteyn, born 160 years ago this week, devoted considerable effort to measuring the proper motion of stars, and he found that most were not traveling randomly, as had been thought, but were heading in one of two opposing directions. Kapteyn’s analysis provided some of the earliest evidence for the rotation of our galaxy. Seen here is NGC 2903, a rotating barred spiral galaxy slightly smaller than our own, 30 million light-years away.